1. Field of Invention
The present invention relates to methods and apparatuses for measuring cylindrical and substantially cylindrical bores and, more particularly, is directed toward methods and apparatuses for measuring the internal volume and nominal diameter of cylindrical and substantially cylindrical bores.
2. Description of the Prior Art
Rock bolt installations are evaluated on their ability to support mine roofs, expressed as the bolt pullout force. This force enables the bolts to resist sagging of the roof rock layers and separation of the immediate ceiling rock. One type of rock bolt with a longitudinally extending bore and slot, which is sold under the trademark Split Set, develops its frictional support against the rock by being forced into an undersized hole, closing down its longitudinal slot. This causes both elastic and plastic deformation of the bolt. Varying the hole diameter changes the amount of deformation, affecting the radial pressure and the resulting frictional force between the bolt and rock.
The pullout strength of a Split Set bolt may be determined by actually loading the bolt until it slips. This is a destructive test, since the support applied to the roof plate and the immediate ceiling rock is released (although frictional support along the length of the bolt is maintained). In order to perform a pull test, the bolt must be installed with an extra washer and spacer to provide a gripping surface for the pullout test collar. As a result, an extra Split Set bolt is installed in a section when a pull test is intended.
To date, evidence that a particular rock bolt installation is well anchored has been circumstantial or very inexact. Proper bolt hole size cannot be assured because of changes in bit wear, strata conditions, and operation of the drill. A need has arisen for an efficient and accurate apparatus and method for determining rock bolt pullout force.